Magnetic resonance imaging (MRI) is a very powerful tool in research and diagnostics. It comprises immerging a body in a static magnetic field B0 for aligning nuclear spins thereof; exposing it to a transverse radio-frequency pulsed field B1 at a resonance frequency known as the “Larmor frequency” for flipping said nuclear spins by a predetermined angle; and detecting a signal emitted by flipped nuclear spins, from which an image of the body can be reconstructed.
There is a trend to move towards higher and higher static magnetic fields in order to improve the spatial resolution of MRI. For example, magnetic fields of 1.5 T (Tesla) are currently used in clinical practice, 3 T is the highest field used in commercial apparatuses and research systems can operate at more than 7 T. However, as the strength of the static magnetic field increases, the wavelength of the radio-frequency pulsed field decreases and its amplitude distribution within the body to be imaged becomes less homogeneous.
Radio-frequency pulsed field inhomogeneities already introduce significant artifacts at 3 T. At 7 T, the Larmor frequency of protons is about 300 MHz, which corresponds to a wavelength around 14 cm in the human brain, i.e. a size comparable to that of a human head. In these conditions, the radio-frequency pulsed field B1 is so inhomogeneous that images e.g. of a human brain obtained with standard techniques can become very difficult to interpret.
The radio-frequency (or “B1”) inhomogeneity problem is so important that it could hinder further developments of high-resolution MRI.
Moreover, the static magnetic field B0 also shows a certain spatial inhomogeneity, which in turn induces artifacts. This effect is also worsened by the current trend of increasing the strength of the magnetic field.
A number of techniques have been developed in order to deal with these inhomogeneity problems.
A first possibility (“RF shimming” technique) consists in using a plurality of antennas to generate the radio-frequency pulsed field B1; by adjusting the amplitude and initial phase on each antenna, it is possible to homogenize the RF field by interference. This technique is disclosed e.g. by document U.S. Pat. No. 7,281,113. Its main drawbacks are the smallness of the volume over which the radio-frequency field is effectively homogenized, the added hardware complexity and the large energy deposited in the body.
The so-called “Transmit-SENSE” technique also uses a plurality of antennas. However, this technique does not aim at homogenizing the instantaneous radio-frequency field, but only the resulting spin flip angle.
In other words, the field may stay inhomogeneous at some given instant, but the temporal variation of the radio-frequency field, thanks to the additional use of magnetic field gradients, finally yields the desired excitation pattern. By writing the solution of the Bloch equation in the linear regime, searching for a solution (phase and amplitude) on each antenna is equivalent to a linear inverse problem. Nevertheless, this method has several disadvantages. The first one is the initial approximation used in the calculation (small flip angle approximation, allowing a linear analysis), leading to a deterioration of the results at higher flip angles. Second, like for the “RF shimming” technique, long measurements of the radio-frequency profile for each antenna need to be performed on each patient before actually being able to compute a solution and obtain a uniform excitation. Third, the numerical problem is of large size and requires a non-negligible calculation time, on the patient exam scale. All these factors make the exam for a patient much lengthier. Finally, there is, like for “RF-shimming”, added hardware complexity.
The “Transmit-SENSE” technique is described e.g. by documents U.S. Pat. Nos. 7,075,302 and 6,828,790.
A completely different approach to the inhomogeneity problems is to use “adiabatic pulses”, i.e. pulses whose amplitude and phase are continuously varied, slowly enough so that the spins, if initially along the effective magnetic field, evolve while staying aligned (or anti-aligned) with that same effective magnetic field. A rotation of the spins can therefore be implemented in a robust way since it is mostly the rate of variation of the field that matters, rather than its amplitude. Like for “Transmit-SENSE”, but unlike for “RE-shimming” techniques, adiabatic pulses can also deal with static magnetic field (B0) inhomogeneities. The adiabatic technique is described e.g. by document U.S. Pat. No. 5,019,784.
However, good homogenization performances require pulses which are either too long, or too powerful, or both, and can therefore turn out to be harmful for a patient. Adiabatic pulses whose duration and power are maintained within practical and safe limits show less than satisfying homogenizing properties.